Thanks for the response.
The user manual says this:-
: Enable this option to force clients to obtain a valid IP address from
DHCP within the specified number of seconds. This prevents clients configured with
a static IP address from connecting to the WLAN. Additionally, if a client performs
Layer 3 roaming between different subnets, in some cases the client sticks to the
former IP address. This mechanism optimizes the roaming experience by forcing
clients to request a new IP address.
It states it should also prevent staticly addressed devices connecting as well- even if not roaming between subnets.
You make a good point- but I'm still not sure why the feature should have the impact I am seeing (within a subnet).